Preheat your oven to 350°F (175°C).

Grease and flour a 9x13-inch baking pan, or two 9-inch round cake pans.

In a large mixing bowl, whisk together the flour, sugar, baking soda, salt, and cocoa powder.

Add the mayonnaise, water, and vanilla extract to the dry ingredients. Mix until well combined and smooth.

Pour the batter into the prepared baking pan(s).

Bake for 30 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before serving.